C 3500 Truck 2WD V8-454 7.4L (1990)
CAUTION: Do not use a meter with an impedance of less than 10 Meg ohms.
1. Set the blower speed to LOW.
2. Insert the positive probe into terminal A (circuit 50) and the negative probe into terminal M (circuit 951). The voltage should read 11.5 to 12.0
volts. (This indicates that the controller is providing a ground to the LO speed fan relay.)
3. Move the negative probe to terminal L (circuit 963). The voltage should be 11.5 to 12.0 volts. (This indicates that the controller is properly
providing a ground for the HI speed fan relay.)
Medium Speed Fan Circuit